Two significant factors associated with radio frequency (RF) antenna design are the antenna resonant point or center operating frequency (also referred to herein as the “resonant frequency”) and the antenna bandwidth or the frequency range over which the antenna design can operate. Since these two factors are very important features of antenna design, they are often specified by various RF communications standards. Whether the RF antenna is used for cellular telecommunication, wireless local area network (WLAN) communication, a Global Positioning System (GPS) receiver, or some other application, the performance of the RF antenna is paramount, and the antenna resonant frequency and the antenna bandwidth are of great importance in this regard.
An RF antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency. This is the frequency at which the capacitive and inductive reactances cancel each other out. At this point the RF antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance. The capacitance and inductance of an RF antenna are determined by its physical properties and the environment in which it is located. Most RF antenna designs are operated around the resonant point. This means that there is only a limited bandwidth over which an RF antenna design can operate efficiently. Outside this, the levels of reactance rise to levels that may be too high for satisfactory operation. Other performance characteristics of the antenna may also be impaired when the antenna is operated away from the resonant frequency.